1. Field
This relates to electronic weighing devices, and more particularly to an electronic weighing device that employs surface acoustic waves to measure weight.
2. State of the Art
Precision electronic weighing devices are widely known in the art and there are many different technologies utilized in these electronic weighing devices. Laboratory scales or “balances” typically have a capacity of about 1,200 grams and a resolution of about 0.1 gram, although scales with the same resolution and a range of 30,000 grams are available. The accuracy of these scales is achieved through the use of a technology known as magnetic force restoration. Generally, magnetic force restoration involves the use of an electromagnet to oppose the weight on the scale platform. The greater the weight on the platform, the greater the electrical current needed to maintain the weight. While these scales are very accurate (up to one part in 120,000), they are expensive and very sensitive to ambient temperature. In addition, their range is relatively limited.
Most all other electronic weighing devices use load cell technology. In load cell scales, the applied weight bends an elastic member which has strain gauges bonded to its surface. The strain gauge is a fine wire which undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight. Load cell scales are used in non-critical weighing operations and usually have a resolution of about one part in 3,000. The maximum resolution available in a load cell scale is about one part in 10,000 which is insufficient for many critical weighing operations. However, load cell scales can have a capacity of several thousand pounds.
While there have been many improvements in electronic weighing apparatus, there remains a current need for electronic weighing apparatus which have enhanced accuracy, expanded range, and low cost.
The previously incorporated applications disclose an electronic weighing apparatus having a base which supports a cantilevered elastic member upon which a load platform is mounted. The free end of the elastic member is provided with a first piezoelectric transducer and a second piezoelectric transducer is supported by the base. Each transducer includes a substantially rectangular piezoelectric substrate and a pair of electrodes imprinted on the substrate at one end thereof, with one pair of electrodes acting as a transmitter and the other pair of electrodes acting as a receiver. The transducers are arranged with their substrates substantially parallel to each other with a small gap between them and with their respective electrodes in relatively opposite positions. The receiver electrodes of the second transducer are coupled to the input of an amplifier and the output of the amplifier is coupled to the transmitter electrodes of the first transducer. The transducers form a “delay line” and the resulting circuit of the delay line and the amplifier is a positive feedback loop, i.e. a natural oscillator. More particularly, the output of the amplifier causes the first transducer to emit a surface acoustic wave (“SAW”) which propagates along the surface of the first transducer substrate away from its electrodes. The propagating waves in the first transducer induce an oscillating electric field in the substrate which in turn induces similar SAW waves on the surface of the second transducer substrate which propagate in the same direction along the surface of the second transducer substrate toward the electrodes of the second transducer. The induced waves in the second transducer cause it to produce an alternating voltage which is supplied by the electrodes of the second transducer to the amplifier input. The circuit acts as a natural oscillator, with the output of the amplifier having a particular frequency which depends on the physical characteristics of the transducers and their distance from each other, as well as the distance between the respective electrodes of the transducers.
When a load is applied to the load platform, the free end of the cantilevered elastic member moves and causes the first transducer to move relative to the second transducer. The movement of the first transducer relative to the second transducer causes a change in the frequency at the output of the amplifier. The movement of the elastic member is proportional to the weight of the applied load and the frequency and/or change in frequency at the output of the amplifier can be calibrated to the displacement of the elastic member. The frequency response of the delay line is represented by a series of lobes. Each mode of oscillation is defined as a frequency where the sum of the phases in the oscillator is an integer multiple of 2π. Thus, as the frequency of the oscillator changes, the modes of oscillation move through the frequency response curve and are separated from each other by a phase shift of 2π. The mode at which the oscillator will most naturally oscillate is the one having the least loss. The transducers are arranged such that their displacement over the weight range of the weighing apparatus causes the oscillator to oscillate in more than one mode. Therefore, the change in frequency of the oscillator as plotted against displacement of the transducers is a periodic function. There are several different ways of determining the cycle of the periodic function so that the exact displacement of the elastic member may be determined.
It is generally known in the art of SAW technology that the frequency range in which the losses are the lowest is not necessarily the frequency range in which the oscillator exhibits the best phase linearity. From the teachings of the previously incorporated applications, those skilled in the art will appreciate that in a SAW displacement transducer such as disclosed in the previously incorporated applications, better phase linearity provides a more linear relationship between frequency and displacement. In the case of a weighing apparatus using a SAW displacement transducer as described in the previously incorporated applications, better phase linearity will result in a more linear relationship between weight and frequency.
It is known in the art of SAW oscillators that changing the topology of the oscillator transmitter and receiver can cause a broader bandwidth of the delay line and that a broader bandwidth results in better phase linearity. It is also known that using a smaller frequency range provides better linearity and that a smaller frequency range can be obtained with a longer delay line. Although these known methods can increase phase linearity in a SAW oscillator, the frequency range in which the best linearity is achieved for a particular oscillator is still not necessarily the range with the lowest losses.
From the foregoing, those skilled in the art will appreciate that in order to enhance the accuracy of a SAW displacement transducer such as that used in a weighing device, it would be desirable to cause the SAW oscillator to oscillate in the range having the best phase linearity.
As disclosed in the previously incorporated applications, weighing accuracy is affected by temperature. The previously incorporated applications disclose a SAW temperature oscillator having a transmitter and receiver on the same substrate. The temperature sensitivity of the load cell disclosed in the previously incorporated applications is approximately 500 ppm of the weight reading per 1° C. temperature change. Accuracy of 100 ppm of the weight reading can be achieved if temperature is measured to within 0.2° C. which represents a shift of about 1 kHz of the SAW temperature sensor. This shift is easy to measure in the short term. The resolution of the SAW temperature sensor is on the order of 0.001° C. However, the long term stability of the SAW temperature sensor can drift more than 1 kHz due to many factors including humidity.
In order to overcome some of these issues, co-owned application Ser. No. 09/775,748 (U.S. Pat. No. 6,448,513) discloses as one aspect the use of a “push oscillator” coupled to the delay line for injecting a strong RF signal at a frequency in the middle of the oscillation mode which exhibits the best phase linearity. The frequency of the “push oscillator” is determined experimentally when the scale is calibrated. The RF signal is injected periodically in short bursts. According to a second aspect of the same patent, the “push oscillator” frequency is generated by mixing the temperature oscillator with an adjustable fixed frequency oscillator. This immunizes the “push oscillator” from the affects of temperature. According to a third aspect of the same patent, a thermistor is provided for long term temperature stability. The SAW temperature sensor is periodically calibrated to the thermistor. According to a fourth aspect of the same patent, the SAW oscillators are not hermetically sealed and the SAW temperature sensor is used to correct the displacement sensor for changes in environmental conditions such as humidity.
Even with these improvements, SAW scales still do not meet certain criteria that are desirable for high accuracy scales. For example, while the zero stability of such SAW scales is in the desirable range of 1:50,000 to 1:100,000 (for a temperature range of 10° C.-40° C.), the stability of the span parameter (the weight reading after having zeroed the scale) is typically as low as around 1:10,000. It is desirable that the span parameter be in the same range (i.e., 1:50,000 to 1:100,000) as the zero stability.
The main cause of this problem is the fact that the process of determining the load for the scale consists of measuring the frequency of the SAW transducer under two conditions—first without load (the zero value) and the other under load from the platform (the weight value). A quality of the SAW scale is that zero stability and the span parameter stability for these two frequencies depends on their values within the pass band of the transducer. The zero stability for every point inside the pass band is very similar, but does have slight variations. As an example, without any load on the platform, the frequency of the delay line oscillator could be 92.9 MHz. Under load it could be 93.1 MHz. In this example the span parameter for a single mode is 200,000 Hz (0.2 Mhz). If the scale utilizes multiple modes, the span parameter is effectively 200,000 Hz times the number of modes of the scale. For five modes, the span parameter of the scale is effectively 1.0 MHz.
The span parameter is also dependent upon temperature. For example, for the exemplary spam parameter described above, the frequency of the delay line oscillator without load for two different temperatures can change (i.e., drift) by 1000 Hz, and the frequency of the delay line oscillator under load for the same two temperatures can change (i.e., drift) by 1050 Hz. This is a difference of 50 Hz and is referred to as absolute span drift. In this example, the relative span drift (absolute span drift/span parameter) is 50 Hz/200,000 Hz (1:4000) (for a single mode), which is considered to be a poor result for a high accuracy scale. If the scale utilizes five modes, the absolute span drift (50 Hz) will be the same, but the full range will be five times larger and as a result the relative span drift will drop to 1:20,000, which is still higher than desired. In addition, this error will appear as a discontinuity and as a linearity distortion at the points of the border between modes.
In addition, given the wide range of temperatures under which industrial scales operate, −20° C. to +60° C., there is the potential for measurement error due to the mismatching of coefficients of thermal expansion (CTE) between the SAW transducer substrate and the material of the load cell. The transducer substrate is bonded to the load cell using a holder which is made from the same material as the remainder of the load cell; typically, a suitable alloy of aluminum. The transducer substrate and the holder material have significantly different CTEs which will subject the bonding line of the materials to a thermal stress. If the temperature changes significantly, the thermal stress between the materials, including the bonding line, causes some change on the zero reading of the scale which is determined by the exact position of the transducer substrate. Because the bonding material has some level of hysteresis and non-repeatability under stress, the shift of the zero reading can be very unpredictable. Various methods are known for bonding materials with mismatched CTEs, including high temperature or pressure bonding, including brazing or diffusion, or machining operations, including drilling holes or riveting. However, the transducer substrate material is not suitable for these kinds of operations because of fragility and high temperature concerns.